You may recall the extensive information
on the Bass and Paur (1985) ozone absorption coefficients presented at both
the Halkidiki and Gottingen Ozone Symposia, and that the Commission had
charged Dr. Bob Hudson, in collaboration with Dr. Carl Mateer, to study
and make recommendations on their application in the WMO Global Ozone Observing
System (GO3OS),

I am glad to inform you that, after
very elaborate studies carried out by Dr. Hudson who also assessed the other
recent measurements of ozone absorption coefficients, a set for use in the
standard Dobson wavelengths was recommended. Further calculations carried
out by Drs. Mateer and Komhyr were necessary and consultations with other
colleagues resulted in the Guidance attached to this letter.

Since the main recommendation to start
calculating and reporting to the WMO-WO3DC
total ozone amount using the new (Bass and Paur) ozone absorption coefficients
from 1 January 1992 was also endorsed by the WMO Executive Council Panel
on Environmental Pollution and Atmospheric Chemistry, this recommendation
is being communicated formally by the Secretary-General of WMO, Professor
G.O.P. Obasi, to the Permanent Representatives of the countries having total
ozone measurements for implementation.

The present letter and attached Guidance
are forwarded to you in order to keep you informed and to facilitate timely
transition to the new 1992 scale.

Finally, I would like to convey the
appreciation of both the Commission and WMO to our colleagues, Drs Hudson,
Mateer and Komhyr for their dedicated efforts towards successful 'completion
of this important task.

With best wishes for the New Year.

Yours sincerely,

(Dr. Rumen D. Bojkov)
Secretary
International Ozone Commission

To : Members of the Ozone Commission

GUIDANCE FOR USE OF NEW OZONE ABSORPTION COEFFICIENTS IN
PROCESSING DOBSON AND BREWER SPECTROPHOTOMETER TOTAL OZONE DATA BEGINNING
1 JANUARY 1992*

1. Background

Early observations with Dobson spectrophotometers,
first employing photocells and later photomultipliers, were processed using
ozone absorption coefficients based on the laboratory data of Ny and Choong
(1933). At the beginning of the International Geophysical Year (1 July, 1957),
new absorption coefficients were adopted for use (Dobson, 1957), this time
employing the laboratory values of Vigroux (1953). Pre-IGY Dobson spectrophotometer
ozone values were converted to the Vigroux scale by multiplying them by 1.36.

Because the Vigroux coefficients gave
inconsistent ozone values from observations on different combinations of wavelengths
(e.g. CD-wavelength observations yielded ozone values nearly 10% smaller than
did AD-wavelength observations), the Vigroux AD-wavelength ozone absorption
coefficients were adopted as standard. Based on a redetermination of the ozone
absorption coefficients by Vigroux (1967) and an examination of the coefficients
by G.M.B. Dobson in light of atmospheric observations made in Canada, the
United Kingdom and elsewhere, a modified set of Vigroux ozone absorption coefficients
was derived which gave highly consistent ozone amounts from observations made
on different wavelengths. These modified coefficients, which preserved the
original Vigroux AD-wavelength coefficient as standard were recommended by
the International Association of Meteorology and Atmospheric Physics (IAMAP)
for use in the WMO Global Ozone Observing System (GO3OS)
beginning 1 January 1968. Rayleigh molecular scattering coefficients used
with the ozone absorption coefficients in processing ozone data were derived
from the Rayleigh Cabannes formula (Dobson 1957).

During the mid 1980's, several research
groups redetermined ozone absorption coefficients in the Hartley and Huggins
bands. In 1988, the International Ozone Commission assigned an action item
to R.D. Hudson and C.L. Mateer to derive and recommend for use improved Dobson
instrument ozone absorption coefficients based on the new laboratory data
and to set a date for their use.

Hudson (1990) recommended use of the
Bass-Paur (1985) ozone absorption coefficients, adjusted to account for the
temperature dependence (as determined by Barnes and Mauersberger 1987) of
the Hearn (1961) 253.7 nm wavelength ozone absorption coefficient. Subsequently,
Mateer (1990) used a "dynamic" approach to compute effective ozone
absorption coefficients for the Dobson instruments. The method not only weighs
the Bass-Paur coefficients with the instrument slit function weights, at an
ozone-weighted mean temperature determined from convolution of appropriate
temperature and ozone profiles but also takes into account solar flux weighting
at the instrument slits. Subsidiary data used by Mateer in obtaining results
were the slit function data for world standard Dobson spectrophotometer 83
determined experimentally by Komhyr (1982); the extra-terrestrial solar spectrum
compiled by Furukawa et al. (1966); the Rayleigh scattering cross-sections
calculated by Bates (1984); the atmospheric temperature profiles in the US
Standard Atmosphere, 1962 and the US Standard Atmosphere supplements,
1966: the standard ozone profiles compiled by Mateer, DeLuisi and Porco
(1980); and the standard ozone profiles compiled by Bhartia et al. (1984)
for use in re-evaluation of NIMBUS 7 SBUV ozone profile data.

The Dobson instrument ozone absorption
coefficients tentatively recommended for use by Mateer (1990) were examined
by Komhyr early in 1990 for consistency in ozone amounts derived from quasi-simultaneous
observations on different wavelengths. Observational data used were those
obtained with Dobson Instrument 83 at Mauna Loa Observatory during summertimes
since 1975 (Komhyr et al. 1989). Using the new coefficients, only small differences
were observed in ozone amounts derived from observations made on different
wavelengths compared with ozone amounts derived from observations on AD-wavelengths.
The range of differences in ozone values derived from observations on A, BD,
B, CD, D wavelengths was only 3.8%. Komhyr noted that the range of differences
could be reduced to 0.9% if the coefficients recommended for use by Mateer,
1990 for B and D wavelengths were increased by 1.3% and 2.0% respectively.
This modification of the B and D coefficients was agreed upon after consultations
with Hudson, Mateer and Bojkov and a decision was made to recommend use of
the new coefficients in processing Dobson and Brewer instrument ozone data
beginning 1 January 1992. This was also endorsed by the WMO Executive Council
Panel of Experts/CAS Working Group on Environmental Pollution and Atmospheric
Chemistry for implementation in the WMO Global Ozone Observing System (GO3OS).

*: Prepared by R.D. Hudson, W.D. Komhyr, C.L. Mateer and R.D. Bojkov on
behalf of the ozone Commission of IAMAP

2. New Dobson Instrument Ozone Absorption Coefficients

The effective ozone absorption coefficients
listed in Table 1 are the Bass-Paur coefficients adjusted to account for the
temperature dependence of the Hearn (1961) 253.7 nm wavelength ozone absorption
coefficient and adapted for use with Dobson instruments by Mateer (1990) but
with the Mateer B and D wavelength coefficients increased by 1.3% and 2.0%
respectively in order to achieve agreement in ozone values to within about
1% from observations made on different wavelengths. Table 1 also lists the
slit weighted average Rayleigh molecular scattering coefficients derived from
the data of Bates (1984). The Bates coefficients are generally considered
to be the best currently available.

(a) The effective ozone absorption coefficients
listed in Table 1 should be adopted as standard for total ozone measurements
with Dobson ozone spectrophotometers;

(b) The Rayleigh scattering coefficients
listed in Table 1 should be adopted as standard for total ozone measurements
with Dobson spectrophotometers;

(c) The standard effective ozone absorption
coefficients apply to the US Standard Atmosphere 1962 and the Bhartia et al.
standard ozone profile for 45°N and 325 DU total ozone. The values mu=2 and
ozone weighted mean temperature -46.3°C are also inherent in these coefficients;

(d) Zenith Blue and Zenith Cloud total
ozone observations should be evaluated using existing empirical cloud charts
that were devised to yield total ozone amounts which agreed with ozone values
determined quasi-simultaneously from direct sun. AD-wavelength observations
[with Vigroux (1953) absorption coefficient 1.388]. Convert to the new scale
by multiplying all values by 0.9143;

(e) These recommendations should be put
into effect 1 January 1992.

Provisional ozone data obtained using
the new absorption and scattering coefficients should continue to be sent
routinely to the WMO world Ozone Data Center in Canada for archiving. Each
station submitting their data should include in the line for comments a message
"processed with 1992 absorption coefficients". These standardized
data sets could be subject to additional improvements in special studies,
e.g through application of seasonal mean temperature profile corrections,
or corrections related to values of mu*X. 3. Converting Past
Data

Errors in the converted data will be
less than 0.3% for 220<550 D.U., 1<3 and 680<1013
mbar. Note that because of the use of improved Rayleigh scattering coefficients,
the correction factor of 0.9743 is slightly larger than the factor 1.388/1.432="0.9693"
derived from the ratio of the old and new AD- wavelength ozone absorption
coefficients.

Since July 1957, a few stations have
published total ozone data from C- and CD-wavelength direct sun observations
using the Vigroux (1953) ozone absorption coefficients scale, utilizing absorption
coefficient values of 0.865 cm-1 and 0.491 cm-1 respectively. To convert such
data to the new 1992 scale (based on the modified Bass-Paur (1985):

Total ozone measurements made with the
Brewer spectrophotometer have been reported to the WMO World Ozone Data Center
(WO3DC) in Toronto using absorption coefficients
adjusted to make the Brewer instrument measurements directly comparable with
those made by the Dobson instrument. The Dobson measurements are based on
the Vigroux scale of absorption coefficients adopted by WMO, IAMAP since 1
January 1968. Effective 1 January 1992, the total ozone measurements made
with the Dobson instrument are to be reported to the WO3DC
based on the ozone absorption coefficient measurements made by Bass and Paur
(1985) as explained above.

In coordination with the conversion of
absorption coefficient scale for the Dobson instrument, the total ozone measurements
made the Brewer instrument are also to be reported to the WO3DC
on the Bass-Paur (BP) scale effective 1 January 1992.

Reporting Brewer ozone data on the BP
absorption coefficient scale is achieved by multiplying the daily average
total ozone values by 0.973.

For direct sun measurements this can be done by dividing the absorption
coefficients (both for ozone and SO2)
by 0.973 and entering them into the instrument constant files.

For zenith sky measurements the daily summarized value is multiplied
by 0.973.

For data submission to the WO3DC
the fact that the ozone measurements are on the BP scale should be confirmed
by including the statement "BP SCALE" on line five of the monthly
data submission record.